Low-Cost Magnetic Stripe Reader Using Independent Switching Thresholds

ABSTRACT

A F/2F waveform generator has a comparator and an analog multiplexer. In a low-cost magnetic card reader application, a magnetic track signal is amplified, filtered, and compared with a threshold signal to create a digital signal output. The analog multiplexer detects changes in state of the digital signal. When a change of state is detected, the analog multiplexer switches among dynamically tunable threshold signals. The selected threshold signal is used for comparison with the magnetic track signal. Switching level detection enables accurate F/2F waveform generation from relatively noisy magnetic track signals, thus improving the robustness of magnetic card readers. The analog implementation eliminates the need for expensive A/D conversion and processing and the design can be readily implemented in a very compact and low-cost package.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority under 35U.S.C. §120 from, nonprovisional U.S. patent application Ser. No.12/221,161 entitled “Low-Cost Magnetic Stripe Reader Using IndependentSwitching Thresholds,” filed on Jul. 31, 2008, now U.S. Pat. No. ______,the subject matter of which is incorporated herein by reference.

TECHNICAL FIELD

The described embodiments relate to magnetic card readers, and moreparticularly to interface circuitry having raw magnetic track signalinterfaces and associated functionality.

BACKGROUND INFORMATION

FIG. 1 (prior art) is a simplified block diagram of a so-called magneticstripe reader 1 as might be part of a common point of sale credit cardterminal device. A card 3 encoded with a magnetic stripe 20, isphysically swiped past a magnetic pick-up unit 8, generating a magnetictrack signal 10. Typically, a F/2F waveform generator 2 then processesthis magnetic track signal 10. This may occur by first amplifying thesmall amplitude magnetic track signal 10 using a gain stage 4 togenerate an amplified track signal 11, then filtering the amplifiedtrack signal 11 using a low-pass filter 5 resulting in an amplified,filtered track signal 12. This signal is then processed by a peakdetector 6 to produce a digital signal 13 that reflects the magneticstripe 20 encoded on the card 3.

FIG. 2 (prior art) further details the signal processing steps performedby a magnetic stripe reader illustrated in FIG. 1. A typical card 3 isencoded with a magnetic stripe 20. This magnetic stripe is a series ofmagnetic pole pairs disposed end-to-end such that magnetic fluxconcentrations are linearly spaced along the magnetic stripe 20. Atypical magnetic stripe 20 is linearly subdivided into a series of equallength bit cells 26 that represent a digital bit string 23. If onemagnetic pole-pair is encoded across a single bit cell, this representsa zero bit 25. If two magnetic pole-pairs are encoded end-to-end acrossa single bit cell, this represents a one bit 24. Viewed over time, thesignal is a sequential superposition of signals of a fixed frequencyrepresenting a zero bit and of signals of twice the fixed frequencyrepresenting a one bit. For this reason, the digital waveform 22 thatresults from reading typical magnetic cards is commonly termed a F/2Fwaveform. Furthermore, the elements used to transform a magnetic tracksignal into digital waveform 22 may be termed a F/2F waveform generator.

FIG. 3 (prior art) represents a first approach to F/2F waveformgeneration in magnetic stripe readers. The magnetic track signal fromthe magnetic pick-up unit 31 is amplified and low-pass filtered beforethe signal is digitized by an analog-to-digital converter 36. Thedigital signal may be further filtered by a digital filter 37 beforebeing processed by a digital peak detector 39 to generate the F/2Fwaveform. The digital approach to F/2F waveform generation has severaldisadvantages. It is large and complex to implement on silicon, leadingto high production cost. Furthermore, both the digital filtering andpeak detection schemes require a significant and undesirably expensivesoftware implementation effort.

FIG. 4 (prior art) illustrates a second approach to F/2F waveformgeneration in magnetic stripe readers. The circuit switches betweendigital high and digital low output values when the magnetic tracksignal crosses predetermined threshold voltages. The threshold voltagesare determined by the gain and operation of analog circuit components.

SUMMARY

A novel integrated circuit has a comparator, a signal amplifier, and ananalog multiplexer. In one embodiment, the signal amplifier is aprogrammable inverting operational amplifier (PIOA) that supplies anamplified signal to the non-inverting input of the comparator. Thecomparator compares the amplified signal with a threshold signal presenton the inverting input of the comparator. The digital output signal ofthe comparator is a control signal (a select input signal) that controlsthe analog multiplexer such that a change in state of the digital signaldetermines which one of a plurality of independently programmablevoltages is coupled by the analog multiplexer to be the threshold signalpresent on the inverting input of the comparator.

In a magnetic stripe reader application, a magnetic track signal issupplied to the PIOA. A first programmable voltage source supplies thereference voltage signal for both the PIOA and the magnetic pick-upunit. A second programmable voltage source supplies a high thresholdvoltage signal to a first data input lead of the analog multiplexer. Athird programmable voltage source supplies a low threshold voltagesignal to a second data input lead of the analog multiplexer. Anon-board processor controls the magnitude of the voltage signal suppliedby each of the voltage sources and controls the gain and offset of thePIOA. The desired values for each of these parameters may be stored inon-board memory.

The comparator switches the state of the digital output when anamplified track signal first crosses a threshold voltage. By switchingthe threshold voltage between a high threshold voltage signal and a lowthreshold voltage signal, a large tunable hysteresis band is introducedin the detection scheme. This permits accurate peak detection from anoisy amplified track signal. By using programmable voltage sources,controlled by an on-board processor, the threshold levels may bedynamically tuned for each magnetic card reader application and/or eachmagnetic card swipe. Dynamic tuning permits optimal peak detection inthe face of environmental noise. This may reduce the number of cardmisreads and may reduce the number of times a user must re-swipe themagnetic card to obtain a successful read. Furthermore, the use ofanalog components to implement the switching level detection schemeenables a particularly compact and low-cost integrated circuitimplementation, thus enabling high performance peak detection in costsensitive applications such as magnetic stripe readers.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 (prior art) is a diagram illustrative of a prior art magneticstripe reader of a type often employed in point of sale card readerdevices.

FIG. 2 (prior art) is a diagram illustrative of the common signalprocessing steps to translate an encoded magnetic stripe into a digitalbit string used by a digital processor.

FIG. 3 (prior art) is a diagram illustrative of a prior art embodimentof a F/2F waveform generator for a magnetic stripe reader employingdigital filtering and digital peak detection.

FIG. 4 (prior art) is a diagram illustrative of a prior art embodimentof a F/2F waveform generator for a magnetic stripe reader employinganalog peak detection.

FIG. 5 is a diagram illustrative of a novel F/2F waveform generator.

FIG. 6 is a diagram illustrative of a F/2F waveform generator circuit inaccordance with one novel aspect.

FIG. 7 is a waveform diagram illustrative of a magnetic track signalfrom a magnetic pick-up unit buried in larger amplitude, high frequencynoise.

FIG. 8 is a waveform diagram illustrative of the signals of FIG. 7,except that in FIG. 8 the magnetic track signal has been amplified andthe high frequency noise has been filtered such that the amplitude ofthe magnetic track signal exceeds that of the noise signal.

FIG. 9 is a waveform diagram illustrative of the signals of FIG. 8,except that FIG. 9 includes the F/2F waveform output of the F/2Fwaveform generator.

FIG. 10 is a simplified flowchart of a method of generating digitalwaveforms in accordance with the novel aspect of FIG. 6.

DETAILED DESCRIPTION

FIG. 5 is a diagram illustrative of a low-cost F/2F waveform generator60. F/2F waveform generator 60 includes a magnetic pick-up unit 61 andan integrated circuit 62. Magnetic pick-up unit 61 functions in aconventional way and as described in the background section of thispatent document. Components 61 and 62 are typically fixed to a printedcircuit board, and the printed circuit board is contained in a suitableenclosure (not shown) with access such that a magnetic stripe can engagemagnetic pick-up unit 61.

In one embodiment, integrated circuit 62 includes a processor core 69,an amount of memory 70 (program and data memory such as, for example,FLASH and/or RAM), a databus 71, a first programmable internal referencevoltage source 75, a second programmable internal reference voltagesource 73, a third programmable internal reference voltage source 74, aplurality of terminals including analog input terminal 76 and referenceterminal 77, an analog multiplexing circuit 66, a programmable invertingoperational amplifier (PIOA) 68, and an analog comparator 67.

By writing appropriate control values into control register(s) 57,processor core 69 can configure and control blocks 73,74, 75, and 68 ofintegrated circuit 62. The lines labeled “C” in FIG. 5 represent thecontrol values stored in the control register(s). For example, processorcore 69 can set the magnitude of a reference voltage signal 59 output bythe first programmable internal reference voltage source 75, can set themagnitude of a high threshold voltage signal 65 output by the secondprogrammable internal reference voltage source 73, can set the magnitudeof a low threshold voltage signal 64 output by the third referencevoltage source 74, can set the gain of PIOA 68, and can set the inputvoltage offset of PIOA 68.

In one novel aspect, signal amplifier 68 amplifies a magnetic tracksignal 58 from a magnetic pick-up unit 61 connected to terminal 76. Theresulting amplified track signal 79 is supplied to a comparator 67 whereit is compared with a threshold signal 63. For example, if the amplifiedtrack signal 79 exceeds the threshold signal 63, then a digital highoutput signal 78 is generated by the comparator. Whereas, if theamplified track signal 79 is less than the threshold voltage signal 63,then a digital low output signal 78 is generated by the comparator. In atypical magnetic card reader application, the digital signal 78 istermed an F/2F waveform as described in the background section of thispatent document. The value of digital signal 78 may be read by processor69 via databus 71.

In another novel aspect, the comparator output lead is also connected tothe control signal input lead of an analog multiplexer 66 that mayselect between a plurality of input signals, for example, a highthreshold voltage signal 65 and a low threshold voltage signal 64. Forexample, if the comparator output signal is a digital high signalvoltage, the analog multiplexer 66 selects the low threshold voltagesignal as the output signal of the analog multiplexer 66. For example,if the comparator output signal is a digital low signal voltage, theanalog multiplexer 66 selects the high threshold voltage signal as theoutput signal of the analog multiplexer 66. The output signal of theanalog multiplexer 66 is the threshold voltage 63 which is compared withthe amplified signal voltage 79.

FIG. 6 is a diagram illustrative of one embodiment of a novel F/2Fwaveform generator circuit 72. F/2F waveform generator circuit 72includes a signal amplifier 68, a comparator 67, and an analogmultiplexer 66. The signal amplifier 68 may be an inverting amplifier 80with a programmable gain and offset. The inverting input lead receivesthe magnetic track signal 85. The non-inverting input receives areference voltage signal 86 that may also be present on the return leadof magnetic pick-up unit 61. The output of the signal amplifier 68 isconnected to the non-inverting input lead of analog comparator 81. Theinverting input lead of analog comparator 81 is connected to the outputlead of analog multiplexer 82. The output of the comparator 81 isdigital signal 78. In a typical magnetic card reader application digitalsignal 78 is termed an F/2F waveform as discussed in the backgroundsection of this patent document.

In one embodiment (shown in FIG. 6), a capacitor 90, for example of 2.7picofarads, couples the output signal 84 of the signal amplifier toreference voltage signal 86. This capacitor 90, for example, provides afirst order roll-off at for example, 500 khz cut-off frequency toattenuate high frequency noise in the amplified track signal.

In another embodiment, an optional low-pass filter 87, either passive oractive, of first order roll-off, or higher order roll-off is placed inthe circuit between the output lead of the signal amplifier 80 and aninput lead of the comparator 81 to attenuate high frequency noise in theamplified track signal. In another embodiment (not shown) signalamplifier 68 is a non-inverting amplifier with programmable gain andoffset.

FIG. 7 is a waveform diagram illustrative of an amplified, magnetictrack signal 79 extracted from a magnetic track signal 58 that isencumbered with large amplitude, high frequency noise. For example, in atypical magnetic card reader application, the magnitude of a magnetictrack signal is for example, ten millivolts peak-to-peak and themagnitude of a noise signal is for example, one hundred millivoltspeak-to-peak.

FIG. 8 is a waveform diagram illustrative of the signals of FIG. 7,except that in FIG. 8 the magnetic track signal has been amplified andthe high frequency noise has been filtered such that the amplitude ofthe magnetic track signal is, for example, 2 volts peak-to-peak and theamplitude of the noise signal is, for example 500 millivolts.

FIG. 9 is a waveform diagram illustrative of the signals of FIG. 8,except that FIG. 9 includes the F/2F waveform output of the F/2Fwaveform generator 78. The F/2F waveform remains at digital low voltageuntil the high threshold reference voltage signal 65 is crossed by theamplified track signal 79 and the F/2F waveform remains at digital highvoltage until the low threshold reference voltage signal 64 is crossedby the amplified track signal 79.

In one novel aspect, the high threshold voltage signals and the lowthreshold voltage signals can be independently tuned to achieve a largeamplitude, dynamically tunable hysteresis band. A hysteresis band is thevoltage difference between a high threshold reference voltage signal 65and a low threshold reference voltage signal 64 implemented at anyparticular time. For example a hysteresis band of 0.5 volts or greatermay be generated. A large amplitude, dynamically tunable hysteresis bandenables robust F/2F waveform generation in the face of an amplifiedtrack signal contaminated by noise. For example, the dynamic tuning ofthe threshold signals may be achieved by the processor 69 readingthreshold values from memory 70 and writing those values to thresholdvoltage generators 73 and 74. In another example, processor 69 mayprocess information regarding signal quality during a swipe and updatethe values of threshold voltage generators 73 and 74 to optimize readperformance. In another example, there may be a plurality of voltagegenerators and the analog multiplexer 68 may select the thresholdvoltage signal 63 from a plurality of available voltage generators basedon the particular card reader application or the conditions of aparticular swipe.

FIG. 10 is a flowchart of a method 150 where an amplified track signalis compared with the current threshold signal 151. If the amplifiedtrack signal is greater than the current threshold signal a digital highF/2F waveform signal is generated 152. If the amplified track signal isless than the current threshold signal a digital low F/2F waveformsignal is generated 153. The resulting F/2F signal is then used toupdate the threshold voltage 154. If the F/2F waveform signal is digitalhigh, then the threshold voltage is updated with a low threshold voltagesignal value 155. If the F/2F waveform signal is digital low, then thethreshold voltage is updated with a high threshold voltage signal value156.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Circuit 62 may be an amount of programmable logic of afield programmable gate array (FPGA) architecture. The overall F/2Fwaveform generator circuit of FIG. 6 has a smaller footprint than theprior art circuit of FIG. 3 that involves more components. It istherefore more cost effective to implement in production. In addition,the analog implementation of FIG. 6 with switching level detection withlarge amplitude hysteresis and dynamically tunable threshold voltagesreduces the software development effort required to develop and tune theprior art circuit of FIG. 3, while maintaining robust peak detection inthe face of noisy signals. As noise levels among different credit cardapplications vary widely, the flexibility to program the high and lowreference threshold voltages independently increases card readerrobustness. The overall component count of circuit 72 is lower thanprior art FIG. 4, resulting in a simpler implementation and thedynamically tunable high and low threshold voltage references enableimproved circuit tuning for specific credit card reader applications.

Although the novel integrated circuit is described above in connectionwith magnetic card reader applications, the integrated circuit seesgeneral usage in signal detection applications, especially where asensor output signal has a low amplitude desired signal contaminated bylarger amplitude, high frequency noise and a digital output based onpeak detection is required. Accordingly, various modifications,adaptations, and combinations of various features of the describedembodiments can be practiced without departing from the scope of theinvention as set forth in the claims.

1-8. (canceled)
 9. A method comprising: amplifying a magnetic tracksignal thereby generating an amplified track signal; comparing theamplified track signal to a threshold signal thereby generating adigital signal; and using the digital signal to select which one of aplurality of programmable voltage sources is supplying the thresholdsignal.
 10. The method of claim 9, wherein an inverting operationalamplifier amplifies the magnetic track signal.
 11. (canceled)
 12. Themethod of claim 9, further comprising: filtering the amplified tracksignal by coupling the amplified track signal to a reference voltagesignal via a capacitor.
 13. The method of claim 9, further comprising:filtering the amplified track signal with a low-pass filter. 14-15.(canceled)
 16. The method of claim 9, wherein an analog multiplexerchanges the threshold signal based on the digital signal. 17-20.(canceled)
 21. The method of claim 9, wherein the plurality ofprogrammable voltage sources includes a low threshold voltage source anda high threshold voltage source, and wherein if the digital signal is adigital high, then the low threshold voltage source supplies thethreshold signal.
 22. The method of claim 9, wherein the thresholdsignal has a voltage magnitude, further comprising: dynamically settingthe voltage magnitude by programming a control register.
 23. The methodof claim 9, wherein the plurality of programmable voltage sourcesincludes a low threshold voltage source and a high threshold voltagesource, and wherein the threshold signal supplied by the high thresholdvoltage source has a voltage that is more than 0.5 volts greater thanthe voltage of the threshold signal supplied by the low thresholdvoltage source.
 24. The method of claim 9, wherein one of the pluralityof programmable voltage sources is selected by supplying the digitalsignal to a control signal input lead of a multiplexer.
 25. A methodcomprising: amplifying a magnetic track signal thereby generating anamplified track signal; comparing the amplified track signal to athreshold signal and thereby generating a digital signal; and selectingwhether the threshold signal is either a high threshold voltage signalor a low threshold voltage signal using the digital signal.
 26. Themethod of claim 25, wherein the selecting either the high thresholdvoltage signal or the low threshold voltage signal is performed bysupplying the digital signal to a control signal input lead of amultiplexer.
 27. The method of claim 26, wherein the multiplexer outputsthe threshold signal.
 28. The method of claim 25, wherein the comparingis performed by an analog comparator.
 29. The method of claim 25,wherein the digital signal is received onto a bus.
 30. The method ofclaim 25, wherein the digital signal is an F/2F waveform.
 31. The methodof claim 25, wherein the digital signal remains a digital low voltageuntil a first voltage of the amplified track signal exceeds a secondvoltage of the high threshold voltage signal, and wherein the digitalsignal remains a digital high voltage until the first voltage of theamplified track signal becomes less than a third voltage of the lowthreshold voltage signal.
 32. The method of claim 25, wherein the highthreshold voltage signal has a first voltage that is more than 0.5 voltsgreater than a second voltage of the low threshold voltage signal. 33.The method of claim 25, wherein the amplifying, comparing and selectingare performed by a field programmable gate array.
 34. The method ofclaim 25, wherein the amplifying is performed by an amplifier having aprogrammable gain.
 35. The method of claim 25, wherein the amplifying isperformed by a programmable inverting operational amplifier, furthercomprising: changing a gain of the programmable inverting operationalamplifier by setting a control register value.